As fine wiring materials for liquid crystal display apparatus, aluminum thin films have been conventionally used; however, in recent years, copper thin films having a characteristic of lower resistance than that of aluminum are attracting attention (see Patent Literatures 1 and 2).
Conventionally, copper has been used as a metal material for patterning printed wiring boards. However, until recently, copper or copper alloys consisting primarily of copper have not been used for the formation of fine patterns and driving-transistor electrodes with a line width of several micrometers or less in flat panel displays. Accordingly, to date, there has been only a limited range of etching technology for copper thin films with a line width of several micrometers or less, suitable for the production of flat panel displays.
When a copper thin film is used as an electrode, it is not that a copper monolayer is used, but metals such as Ti, Mo, MoTi should be used as a contact layer or barrier layer in order to improve the contact property with glass substrates and to block the dispersion of copper. In such cases, generally, laminated films of Ti/Cu/Ti, Cu/Ti, Mo/Cu/Mo, Cu/Mo, MoTi/Cu/MoTi, and Cu/MoTi are attempted to be used for electrode.
Since dry etching of Cu is difficult, etching of Cu/Mo is carried out by etching using a peroxide such as hydrogen peroxide and peroxosulfuric acid as an oxidant, and that of Cu/Ti is carried out by etching using a peroxide such as hydrogen peroxide and peroxosulfuric acid as an oxidant as well as by an etching method wherein Cu and Ti are subjected to two types of wet etching, or by an etching method wherein Cu is wet-etched and Ti is dry-etched (Patent Literatures 3 and 4).
However, etching solutions using these peroxides have the following problems: i) because it contains peroxide, the etching solution becomes unstable and the supply by one solution becomes difficult in some cases, ii) the life of etching solution is short because decomposition of peroxide is accelerated due to Cu ions dissolved in the etching solution, iii) the peroxide accumulated in dead space or in waste liquid has a risk of explosion, iv) in dry etching, particles tend to generate easily and yield may be reduced, and etching devices with depressurized specification are expensive.
Therefore, an etching process enabling excellent etching patterns without the use of an etching solution comprising peroxide, has been awaited.
An excellent etching pattern here refers to an even etch, such as that the etching accuracy of the line width of etched metal is high, the shape of pattern edge is sharp, and a tapered-shape pattern is formed. When the shape of a pattern edge is irregular and not sharp, then problems such as braking of wires and short-circuiting occur; when a tapered-shape pattern is not obtained, then the step coverage in the next step of thin-film formation deteriorates.
In wet etching of aluminum that has been conventionally used as a wiring material, there is a method using an etching solution of phosphoric acid, nitric acid and acetic acid. However, when such an etching solution is attempted to be used for metals other than aluminum, various factors including etch rate, corrosion potential, contact angle of etching solution with a resist and glass, and dispersion rate affect in a complex manner, making it difficult to obtain patterns having a tapered shape; therefore, application of this etching solution to the metals other than aluminum was only possible for limited objectives under restricted conditions.
The applicant of the present invention has found a method of etching a metal thin film of monolayer consisting primarily of silver, by using a specific composition of mixed acid of phosphoric acid, nitric acid and acetic acid (see Patent Literature 5); however, this method aims etching of the metal thin film of monolayer consisting primarily of silver, which is used as a material for repeller of reflective and semi-transmissive liquid crystal displays; and the method has not been examined for etching copper-laminated films for driving-transistor electrodes and fine patterns of flat panel displays.
Regarding laminated films of silver, a method for etching a laminated film consisting of silver or a silver alloy, in particular a laminated film of silver alloy and molybdenum, using a mixed acid of phosphoric acid, nitric acid and acetic acid has been disclosed (see Patent Literature 6). However, the method described in this literature needs to flow the etching solution to satisfy appropriate conditions, so as to adjust the etch rate of the silver alloy and molybdenum, thus requiring efforts to adjust the conditions, and since the etch rate under the flowing condition of mixed acid largely depends on the material properties of silver alloy and molybdenum, it is impossible to apply this method to other metals without modification.
Moreover, there is a report of an etching solution composition to simultaneously etch a single film consisting of copper or copper alloy and multiple films of double layers or more consisting of said metals, using an etching solution composition comprising phosphoric acid, nitric acid and acetic acid (see Patent Literature 7); however, the layer of “copper alloy” in this literature is only exemplified by copper oxide (I) (CuO), and no concrete description is substantially made for alloys of copper with other metals; moreover, this literature does not relate to the control of taper angles, which is a very important factor in fine patterning.
Furthermore, a film of copper oxide (I) (CuO) has a problem in that its contact property with substrates deteriorates due to reduction of the oxide film, caused by hydrogen plasma treatment performed in the production process of TFT in the production process of flat panel displays.
As mentioned above, etching of laminated films involves influence of not only the difference in the etch rate between layers of metals and metal alloys, but also influence of cell action due to the difference in corrosion potential between layers; accordingly, it is very difficult to predict whether or not good pattern shapes can be obtained by etching, based on the etching solution and the metals used in the laminated films.
Other than the above, with respect to laminated films containing copper and copper alloys, various kinds of laminated films have been investigated as a candidate of next-generation films, including laminated films wherein copper alloys are Cu—Mo, Cu—Ti, Cu—Ca, Cu—Mg, Cu—Ca—O, Cu—Mg—O, Cu—Al, Cu—Zr, Cu—Mn, Cu—Ni—B, Cu—Mn—B, Cu—Ni—B, Cu—Si, Cu—Al, Cu—Mo, Cu—Al, Cu—Mg—B, Cu—Ti—B, Cu—Mo—B, Cu—Al—B, Cu—Si—B, Cu—Mg—Al, Cu—Mg—Al—O, etc., as well as laminated films containing copper and copper oxides (CuO); however, none of them has been found to be practically satisfactory, and early development of technologies for the fabrication of fine patterns has been awaited.